Grade powders are pressed and sintered to form cemented carbide articles such as drill tools, cutting bits and wear parts. Grade powders comprise a mixture of fine powders of metal carbides and a binder metal. A wax binder may be included in the grade powder to promote the flowability of the grade powder into die cavities, to aid in pressing by a lubricating action, and to impart sufficient green strength to permit handling after pressing.
Prior to sintering, the wax binder is typically removed during the step known as "dewaxing" by heating in a furnace at a temperature of from about ambient to 500.degree. C. in a protective atmosphere or vacuum.
As set forth in U.S. Pat. No. 4,070,184 to Scheithauer et al. a process is described wherein water is used as a milling fluid and a water soluble, long chain polyvinyl alcohol is added to the milled slurry of metal carbide and binder metal after milling but prior to spray drying. According to this process, the wax-like binder is water soluble or compatible with water to ensure adequate distribution of the wax in the grade powder.
Paraffin type waxes which are insoluble in water are typically incorporated into grade powders by use of an organic solvent which dissolves the paraffin wax so that good dispersion of the wax in the powder is assured. The organic fluid is removed by several drying methods to give a wax containing grade powder. Due to the flammability of organic solvents, if spray drying is used, a closed cycle spray drying system is required which utilizes an oxygen free atmosphere such as nitrogen. Generally, this system is characterized by a high initial cost as compared to a water system which can be an open system.
In the water system, water is the milling liquid and a water soluble binder is utilized. The water system is clearly desirable from a safety standpoint as compared to the organic system. However, the water system is indesirably limited to the types of waxes that can be used since the waxes must be compatible with the system to adequately disperse the wax in the powder for achieving desired powder properties.
It is desirable to develop systems for waxing grade powders which permit more flexibility in the utilization of waxes.